Electric blasting cap having increased safety against unintentional initiation

ABSTRACT

The present invention concerns an electric blasting cap having increased safety against unintentional initiation, said blasting cap presenting a bridge wire and in contact with said bridge wire a fuse head composition which latter is surrounded by a protective tubing of dielectric material.

United States Patent Olsson Feb.8, 1972 [541 ELECTRIC BLASTING CAPHAVING INCREASED SAFETY AGAINST UNINTENTIONAL INITIATION [72] inventor:Per 0. I. Olsson, Gyttorp, Sweden [73] Assignee: Nltro Nobel AB,Gyttorp, Sweden [22] Filed: Oct. 1, 1968 211 App]. No.: 764,069

[30] Foreign Application Priority Data Oct. 6, 1967 Sweden ..l3698l67[52] U.S.Cl ..l02/28 [51] Int Cl [58] FieldofSearch ..102/28, 28M

[56] References Cited UNITED STATES PATENTS 2,995,086 8/1961 Scott..l02/28 M Ball 102/28 M Miller 102/28 M Primary ExaminerVerlin R.Pendegiass AttorneyFred C. Philpitt [57] ABSTRACT The present inventionconcerns an electric blasting cap having increased safety againstunintentional initiation, said blasting cap presenting a bridge wire andin contact with said bridge wire a fuse head composition which latter issurrounded by a protective tubing of dielectric material.

2 Claims, 1 Drawing Figure ELECTRIC BLASTING CAP HAVING INCREASED SAFETYAGAINST UNINTENTIONAL INITIATION Blasting caps of this type arepreviously known (vide Swedish Pat. specification No. 143,680). Theprotection against static electricity is achieved by surrounding thefuse head composition, but not the exposed parts of the fuse head alongits whole length by an open-ended tube made of a soft dielectricmaterial which engages around the fuse head composition.

A disadvantage of the known blasting cap is stated in the Pat.application on page 3, left column, lines 9-16.

The known embodiment of the blasting cap does thus not provide forcomplete safety since charges of static electricity can causeconsiderably higher potential differences.

The object of the present invention is to render the known electricblasting cap still safer than has hitherto been possible.

It has now been surprisingly found that this object can be I realized ifthe firing impulse of the fuse head is increased to a value higher thanthe values normally applied, which are in the order of some fewmillijoules per ohm. More particularly, it has been found that thisresult can be achieved for impulse values which lie above 10 millijoulesper ohm, preferably between 50 and 150 millijoules per ohm.

The value of the firing impulse is determined by the dimensions of thebridge wire and the material from which it is made, and to a certaindegree by the fuse head composition. Setting of the firing impulse to acertain desired value is therefore easily accomplished by one havingnormal skill in the art.

It is true that electric blasting caps having a firing impulse valueexceeding the stated limit are known to the art but these caps are notprovided with the protective tube. Such blasting caps, however, havebeen found insufficiently safe against unintentional initiation due tostatic electricity. The conclusion must be drawn from this that anincrease in the firing impulse does not generally afford any decisiveimprovement in the safety of an electric blasting cap against initiationdue to discharges of static electricity. Consequently, the result gainedby combining the teachings of the above Swedish specification Tests havebeen made to support these assertions, the results of which are givenbelow.

lt can be said against the technical utility of the present inventionthat the necessary firing impulse value demands an increase in therequirement of energy, when the blasting caps are fired by means of acurrent impulse through the lines, since requisite pulse energy perblasting cap in this instance is the product of its resistance andfiring impulse value. However, it has been found that even an increasein the firing impulse value to some tens of millijoules per ohm affordssafety against high discharges of static electricity which can beconceived to take place in practice. Firing apparatus constructed tooperate at such a magnitude are technically fully acceptable.

According to a preferred embodiment of the invention the firing impulsevalue therefore lies between 50 and 150 millijoules per ohm.

According to a further embodiment of the invention the protective tubeis not attached to the fuse head in accordance with the teachings of theabove patent specification, but freely inserted in the blasting cap.This design of the protective tube is technically advantageous as thetube can be mounted more easily when mass-producing the blasting cap andas the risk of transferring substances deleterious to the fuse headcomposition from the tube to this composition decreases. Neither needthe choice of material be restricted to soft materials as in the abovePatent specification, but that a rigid material may be used, which iseconomically more advantageous.

The invention will be more closely described in the following withreference to accompanying drawing which shows a diagrammatic view of ablasting cap.

According to the drawing the blasting cap comprises a metal casing l, inwhich is located an explosive charge 2, a charge 3 of a primer and apyrotechnical delay charge 4 in a thick walled tube 5. The casing 1presents at the top a crimped portion 6 which engages a sealing plug 7through which two insulating wires 8, 9 are passed. Connected to the twowires is a fuse'head element which comprises two legs 10, ll, fixedrelative to one another by means of a strap 12. The ends of the legs areconnected with a bridge wire 13 which is embedded in a fuse headcomposition 14. The composition 14, but not the exposed portions of thefuse head, is surrounded along its whole length by a protective tube 15which is freely inserted in the casing of the blasting cap.

to prove the technical efi'ect of the invention laboratory tests weremade in which charged capacitors were discharged, firstly throughblasting caps designed according to the invention and secondly throughblasting caps in which either the protective tube or the increase infiring impulse had been omitted. When effecting the tests the metalcasing of the blasting caps were connected to the one terminal of thecapacitor whereas the other terminal, subsequent to being charged tocertain voltage values, was brought into contact with'one of theconducting lines of the blasting cap. Spark breakdown was thus obtainedin the blasting cap so that the capacitor discharged, despite the factthat the two conducting lines were separated by the insulating sealingplug of the blasting cap. The following test results were obtained witha capacitor having acapacitance of 1,000 pF discharged to voltage 10kv.,

which values can be conceived to appear during practical applications:

Type of blasting cap Relative detonating frequency 1. according toSwedish Pat. specification 143,680 with a firing impulse value ofapprox. 3.5 millijoules p'er ohm Approx. 50 k 2. according to theinvention with a firing impulse value of approx. lOO millijoules per ohmO b For the sake of comparison a number of tests were made with blastingcaps having a firing impulse value of approximately millijoule per ohm,all of which lacked protective tubes around the fuse head. A relativefrequency of initiation which lay between 1 and 10 percent was obtainedhereby.

The strong reduction in firing frequency (from 50-0 percent) achievedaccording to the invention must,.against the background of the resultobtained on comparison tests, be considered surprising to one havingnormal skill in the art.

What I claim is:

I. An electric blasting cap having increased safety againstunintentional firing whichcomprises:

a. a metal casing,

b. an explosive charge, primer and pyrotechnical delay charge positionedadjacent one end of said casing,

c. a sealing plug positioned adjacent the other end of said casing,

d. two insulated conductor wires passing through said sealing plug,

e. an intermediate space within said casing located between 1. saidsealing plug and 2. said explosive charge, primer and pyrotechnicaldelay charge, space,

a fuse head assembly partially filling said intennediate space said fusehead assembly including a fuse head composition and a bridge wireembedded therein,

g. bare electric conductors in said intermediate space, said bareelectric conductors interconnecting said two insulated conductor wiresthat pass through the sealing plug with the bridge wire embedded in thefuse head composition,

h. said electric blasting cap presenting a firing impulse of at least 10millijoules per ohm, and

i. a protective tube of dielectric material freely positioned space.

within said intermediate space and spaced annular-1y from 2. An electricblasting cap as described in claim 1 wherein said fuse head assembly,the firing impulse value is in the range of from about 50 to 150 j. thelongitudinal dimension of said protective tube being millijoules P lessthan the longitudinal dimension of said intermediate 5

1. An electric blasting cap having increased safety againstunintentional firing which comprises: a. a metal casing, b. an explosivecharge, primer and pyrotechnical delay charge positioned adjacent oneend of said casing, c. a sealing plug positioned adjaCent the other endof said casing, d. two insulated conductor wires passing through saidsealing plug, e. an intermediate space within said casing locatedbetween
 1. said sealing plug and
 2. said explosive charge, primer andpyrotechnical delay charge, space, f. a fuse head assembly partiallyfilling said intermediate space, said fuse head assembly including afuse head composition and a bridge wire embedded therein, g. bareelectric conductors in said intermediate space, said bare electricconductors interconnecting said two insulated conductor wires that passthrough the sealing plug with the bridge wire embedded in the fuse headcomposition, h. said electric blasting cap presenting a firing impulseof at least 10 millijoules per ohm, and i. a protective tube ofdielectric material freely positioned within said intermediate space andspaced annularly from said fuse head assembly, j. the longitudinaldimension of said protective tube being less than the longitudinaldimension of said intermediate space.
 2. said explosive charge, primerand pyrotechnical delay charge, space, f. a fuse head assembly partiallyfilling said intermediate space, said fuse head assembly including afuse head composition and a bridge wire embedded therein, g. bareelectric conductors in said intermediate space, said bare electricconductors interconnecting said two insulated conductor wires that passthrough the sealing plug with the bridge wire embedded in the fuse headcomposition, h. said electric blasting cap presenting a firing impulseof at least 10 millijoules per ohm, and i. a protective tube ofdielectric material freely positioned within said intermediate space andspaced annularly from said fuse head assembly, j. the longitudinaldimension of said protective tube being less than the longitudinaldimension of said intermediate space.
 2. An electric blasting cap asdescribed in claim 1 wherein the firing impulse value is in the range offrom about 50 to 150 millijoules per ohm.